Reminiscent of September of 2005, 2011 and 2019, Monsoon 2021 to make a gain before calling it a day! Soon-to-be baked Karachi should brace for hit-or-miss thunderstorms, monsoon withdrawal surely delayed due to vigorous but temporary activities in the Bay and the Arabian Sea. SST and –IOD still unfavorable but brief halt of westerlies in the north can have a profound and positive effect in allowing thunderclouds to cross into the south before the western winds return again.
This year the probability of materialization was quite low as compared to the monsoons of 2011-2014 and 2017-2020. In short, despite being a normal monsoon, this was an unusual season in the sense it started quite actively but in between it dried up and may end actively.
We had predicted since June 4 that this will ‘largely’ be a normal monsoon season with few above normal events. We witnessed areas in North-East and North Punjab experiencing one to two waves of extremely heavy rain (Toppers July Stats by GoP- Sialkot 554.9 mm followed by Islamabad 450 mm). Month of July was overall normal, the month of August was also normal in the North (few stations in reported above normal events) but below normal in the South. Monsoon onset in North happened by the end of 2nd week of July as stated by PWP on June 20 (upgraded on June 25). While onset happened in South including Karachi around the third week for July (July 10 to 17) which was also correctly predicted. Under the view of normal conditions which later deteriorated in August, we forecasted a normal withdrawal process as well in South (29th August till 4th August) and North (5th September till 11 September) citing unfavorable SST conditions in the Arabian Sea and a high pressure over the west of Pakistan.
Due to the dissipation of a prolonged high pressure over south-western parts of Pakistan and adjoining Persian Gulf which has allowed the monsoon axis to shift south, it’s evident that the withdrawal is not going to happen within those time frame. What effect SST and negative Indian Ocean Dipole has is yet to be seen on the upcoming spells which were also an obstacle in August especially in the southern areas. However, unlike in the month of August, there won’t be any major westerlies this time from September 1 till September 6. Therefore, monsoon axis shifting towards the south appears very likely. This can allow the easterlies to penetrate far west again hence we have delayed the withdrawal by 1 to 2-weeks. Conclusive timeframe of withdrawal will be issued in the coming days.
Furthermore, as monsoon retrieves backwards as it did in August, moving-in into territories again isn’t as straightforward especially in the areas that are at the tail end of the monsoon and it largely depends on the intensity of the rain system as well.
We had already mentioned on August 21 about ‘Chances – 55% of a moderate to strong monsoon wave in the north during the end of the last week in August.’ This spell is ongoing and may continue till August 31/1st September (+1,-1 day error) with intervals. Next wave of monsoon spell also moderate to strong (chance at 70%) likely on Sept 7/Sept 8 (+1 day error)
A possibility (25%) was also mentioned on August 21 about a brewing low pressure in the Bay of Bengal. The probability has exponentially increased rapidly at 95%. Monsoon Axis can shift south in the next 36 to 48 hours towards Sindh province. Moisture from upcountry will start to penetrate towards the central and southern areas.
An upper air cyclonic circulation can form near Gujarat and adjoining Arabian Sea leading to the pull effect ahead of the Bay of Bengal weather system. There are three scenarios now;
- The system crosses into the Arabian Sea and moves towards the Balochistan coast.
- The system crosses into the Arabian Sea and stalls at the Gujarat coast and adjoining Sindh coast.
- The system stays over Gujarat and recurves back into India due to an expected formation of a new weather system in the Bay of Bengal.
In Karachi, the temperature will range from 35 °C to 38 °C in the coming week. Hottest days being August 31/September 1 or September 2. Worth noting, there is a huge contrast among numerical models for precipitation.
Two peaks are being observed. 1st chances of rain with isolated heavy falls in Karachi (possibility at 65%) on September 1/ September 2 (+1,-1 day error) – 30 mm to 75 mm. And then another peak (possibility at 55%) on September 3 (+1 day error), similar rain stat.
Watch for urban flooding due to the fragile infrastructure of the city. If Scenario 1 and 2 happen then sea conditions along the Sindh coast can be rough as well. 30% possibility of another monsoon spell in the month of September.
September rains were rare once but not now
In the past, the monsoon of 2005, 2011 and 2019 have caused heavy downpours in Karachi. In 2005, more than 80 mm rain was recorded in 48 hours from September 10 to September 11 in 2005. 140 mm was recorded in 24 hours on September 13, 2011. While in 2019, there were two spells of monsoon rains in the 1st week (officially 40 mm) and at the end of September (officially 20 mm, might had been much heavier in the south due to the presence of an Arabian sea storm which later became Cyclone Hika and a low pressure over eastern India from the Bay of Bengal). Wettest September was in 1959 in which a record monthly precipitation of 315.7 mm was recorded.